comm: Handle duplicated flags and output-delimiter correctly

[BenWiederhake]

This PR implements and tests correct handling of repeated arguments and flags.

This was non-trivial because GNU comm behaves a bit weirdly around repeated arguments for the output-delimiter:

$ comm -123 --out=X --out=X a b  # Succeeds
$ comm -123 --out=X --out=Y a b  # Fails
comm: multiple output delimiters specified
[$? = 1]
$

This PR mostly mirrors this behavior, however it cannot be done perfectly so, as indicated by the new (and ignored) test output_delimiter_multiple_different_prevents_help: Preventing --help later in the command-line because some arguments are semantically contradictory seems like a bad idea.

Note that this quirk cannot be reproduced (sensibly) with uutils-args, because Options::apply cannot indicate failure.

However, I think this difference is firmly in the territory of "irrelevant for compatibility".

This is work towards #5998.

[BenWiederhake]

Changes since last push:

• Just rebased on current main. Looks like I created the PR while the repo was in the middle of a release cycle, which caused some weird inconsistent version numbers.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[cakebaker]

Good work, thanks :)